There is an apparatus that is used as an intrusion sensor, in which laser light is injected into an optical fiber to detect acoustic waves reaching each longitudinal locations along the optical fiber (see, for example, Patent Document 1). The apparatus performs the intrusion sensing by detecting, at each position along the optical fiber, intensity variations of the Rayleigh backscattered light (hereinafter referred to as Rayleigh scattered light) responsive to pulse light from the laser light source.
A method using distributed acoustic sensing, which may be abbreviated as “DAS” in some cases below, has been traditionally known as a technique for the above-mentioned intrusion sensing or for sensing oil and gas wells (see, for example, Non-Patent Document 1). The technique utilizes change in intensity of Rayleigh scattered light as mentioned above, and is called optical intensity-based DAS referred to as “DAS-I” in abbreviation. Since DAS-I utilizes intensity oscillation of Rayleigh scattered light due to an acoustic wave, the signal processing is simple in itself but its sensitivity widely depends on positions, thus limiting the acoustic detection performance. Meanwhile, another technique that utilizes change in phase of Rayleigh scattered light has been recently put into practical use, which is called optical phase-based DAS referred to as “DAS-P” in abbreviation. In DAS-P, since a spatial integral of an acoustic wave represents the phase of Rayleigh scattered light, a spatial differentiation of the phase is required. Moreover, the phase is measured in a wrapped form from the nature of phase; hence, unwrapping (representing a phase in a continuous form) is also required. For that reason, DAS-P, although the signal processing therefor is more complicated than that for DAS-I, has an advantage of being able to reproduce the acoustic waveform accurately.
Although any of the above distributed acoustic detection techniques is used, two kinds of noise: phase noise in the laser light and observation noise in the receiver, significantly affect the acoustic detection performance, which are factors degrading the detection performance. Moreover, an influence of polarization also needs to be taken into account. Among these factors, the influence of observation noise can be suppressed by increasing the signal-to-noise (SN) ratio of the signal using a pulse compression technique (see, for example, Patent Document 2). Furthermore, the influence of polarization can be suppressed by detection such as using a polarization diversity heterodyne detector.